In the Claus process, elemental sulfur is produced by reacting H.sub.2 S and SO.sub.2 in the presence of a catalyst. The Claus system uses a combustion chamber which, at 950.degree.-1,350.degree. C., converts 50 to 70% of sulfur contained in the feed gas into elemental sulfur. Sulfur is condensed by cooling the reaction gas to a temperature below the dew point of sulfur, after which the remaining gas is heated and further reacted over a catalyst. Normally, the gas passes through at least two such Claus catalyst stages.
The different stages of the process may be represented by the following equations: EQU H.sub.2 S+3/2O.sub.2 .fwdarw.SO.sub.2 +H.sub.2 O (I)
2H.sub.2 S+SO.sub.2 .fwdarw.3S.sub.n +2H.sub.2 O (II)
The overall reaction is: EQU 3H.sub.2 S+3O.sub.2 .fwdarw.3S.sub.n +3S.sub.n +3H.sub.2 O (III)
Below 500.degree. C., the symbol n has a value of approximately 8.
The final Claus exhaust gas still contains small amounts of H.sub.2 S, SO.sub.2, CS.sub.2, carbon oxysulfide, CO, and elemental sulfur in the form of a vapor or mist. The exhaust gas can be subjected to post-combustion to convert substantially all sulfur species to sulfur oxides (SO.sub.2 and SO.sub.3), which are then emitted into the atmosphere.
Sulfur emitted as sulfur oxides ("SO.sub.x ") into the atmosphere with the exhaust gas may amount to 2-6% of the sulfur contained in the feed gas in the form of H.sub.2 S. In view of air pollution and the loss of sulfur involved, further purification is imperative.
Claus aftertreatments have been developed. These are carried out after the last Claus stage or after the post-combustion. These aftertreatments are, however, complicated and expensive or inadequate.
On aftertreatment, carried out before post-combustion, seeks to achieve by catalytic conversion as complete a reaction as possible between H.sub.2 S and SO.sub.2. The reaction temperature is lowered to below the condensation point of sulfur, whereby the reaction equilibrium corresponding to equation II is shifted to form sulfur. A distinction is made between dry processes using alternating reactors in which the catalyst is intermittently charged with sulfur and discharged, and processes where H.sub.2 S and SO.sub.2 react in a high-boiling catalyst-containing liquid to form elemental sulfur which is drawn off continuously as a liquid product.
Unfortunately, in these processes any deviation from the optimum H.sub.2 S:SO.sub.2 ratio in the Claus exhaust gas results in a reduced sulfur yield. No appreciable conversion of sulfur compounds such as COS and CS.sub.2 occurs. Sulfur recovery efficiency of Claus using this form of aftertreatment is limited to 98-99%. Cyclic operation, with alternating reactors, requires at least two reactors and much valves and piping.
A second aftertreatment catalytically hydrogenates SO.sub.2 and S with H.sub.2 and CO while COS and CS.sub.2 are simultaneously hydrolyzed with H.sub.2 O into H.sub.2 S which can be treated conventionally.
Hydrogenation/hydrolysis does not require a stoichiometric H.sub.2 S/SO.sub.2 ratio in the Claus exhaust gas. It almost completely converts COS and CS.sub.2 so that sulfur yields of more than 99.8% can eventually be obtained. This process incurs high capital expenditures for elaborate apparatus. It also consumes substantial energy. Recycle of H.sub.2 S reduces the Claus system capacity, while the production of waste water containing harmful constituents presents additional problems. In addition, the treatment (such as amine absorption) used to remove H.sub.2 S is generally ineffective for removing unconverted COS and CS.sub.2. Total emissions of reduced sulfur species are typically around 10 ppm by volume with this after treatment.
A third aftertreatment oxidizes all sulfur compounds into SO.sub.x which is then further processed. Thee processes are downstream of the post-combustion and therefore independent of the mode in which the Claus system is run. There are also dry processes, where SO.sub.2 is adsorbed and returned to the Claus unit or processed to form sulfuric acid, and wet processes, where SO.sub.2 is removed by absorptive scrubbing and further processed. For complete oxidation of COS and CS.sub.2 and CS.sub.2, the energy requirements are high and following the after-combustion, very large exhaust gas flows have to be treated.
The equilibrium conversion of the Claus reaction (equation II) may be improved by condensing out part of the water in the gas. The gas is then reheated and charged to another Claus stage to form elemental sulfur. This produces waste water which is highly corrosive due to the formation of thiosulfuric acid, polythionic acids and sulfurous acid. Processing of such waste water is expensive. Unavoidable formation of deposits of elemental sulfur also occurs during H.sub.2 O condensation. Moreover, there is no conversion of COS and CS.sub.2 so the maximum recovery of sulfur is about 98%. As a result of these disadvantages, this process has not been used on a commercial scale.
Where the aftertreatment involves conversion of all sulfur compounds into hydrogen sulfide, it is also known to oxidize part of said hydrogen sulfide with air into SO.sub.2 or to convert part of the sulfur produced into sulfur dioxide and thereafter catalytically to convert the remaining hydrogen sulfide with sulfur dioxide at 125.degree.-150.degree. C. in fixed-bed reactors into sulfur. The sulfur loaded catalyst is regenerated by passing hot oxygen-free gases containing hydrogen sulfide through the catalyst. This avoids the disadvantages associated with the first type of aftertreatment, such as dependence on H.sub.2 S/SO.sub.2 ratio and COS/CS.sub.2 content in the Claus exhaust gas. Disadvantages of this process are the high capital cost and the higher H.sub.2 S+SO.sub.2 input concentration for the low-temperature reactor caused by the admixture of a separately produced flow of SO.sub.2. The maximum conversion overall efficiency obtainable with this process approaches 99%.
An aftertreatment process which oxidizes all sulfur compounds into SO.sub.2 is exemplified by Groenendaal et al. in U.S. Pat. No. 3,764,665 which issued on Oct. 9, 1973. This patent disclosed a process for removing sulfur oxides from gas mixtures with a solid acceptor for sulfur oxides wherein the solid acceptor is regenerated with a steam-diluted reducing gas and the regeneration off-gas is fed to a Claus sulfur recovery process. The improvement comprises cooling the regeneration off-gas to condense the water vapor contained therein, contacting the cooled off-gas with a sulfur dioxide-selective liquid absorbent, passing the fat liquid absorbent to a buffer zone and then to a stripping zone wherein the absorbed SO.sub.2 is recovered from the liquid absorbent and is supplied to the sulfur recovery process. By operating in this manner, fluctuations in the sulfur dioxide concentration of the regeneration off-gas were leveled-out and a relatively concentrated sulfur dioxide stream was supplied to the sulfur recovery process at a substantially constant rate.
Although this process supplies relatively concentrated sulfur dioxide to the sulfur recovery process at a substantially constant rate, the off-gas must be cooled and the fat liquid absorbent must be transferred to a buffer zone before the absorbed SO.sub.2 can be stripped. Therefore, what is needed is a simpler process whereby these steps are eliminated and energy costs reduced.